Tread for a tire

ABSTRACT

A tire tread comprising formed, at least in part, of a rubber composition based on at least (phr: parts by weight per hundred parts of elastomer):  
     (i) a diene elastomer;  
     (ii) more than 50 phr of an inorganic filler as reinforcing filler;  
     (iii) between 2 and 15 phr of an (inorganic filler/diene elastomer) coupling agent;  
     (iv) between 1 and 10 phr of a methylene acceptor, and  
     (v) between 0.5 and 5 phr of a methylene donor.  
     This tread has, after mechanical running-in of the tire for which it is intended (“auto-accommodation”), a rigidity gradient which increases radially from the surface towards the inside of the tread. Use of such a tread for the manufacturing or recapping of tires. Tires comprising a tread according to the invention, in particular of the snow or ice type (“winter” tires).

[0001] The present application is a continuation of InternationalApplication No. PCT/EPO1/08566, filed 25 Jul. 2001, published in Frenchwith an English Abstract on 7 Feb. 2001, under PCT Article 21(2), whichclaims priority to French Patent Application No. FR00/10094, filed 31Jul. 2000.

[0002] The present invention relates to treads for tires and to rubbercompositions used for the manufacturing of such treads.

[0003] It relates more particularly to treads for tires having a lowrolling resistance, reinforced majoritarily by reinforcing inorganicfillers such as siliceous or aluminous fillers, these treads beingintended in particular for tires fitted on vehicles such as motorcycles,passenger cars, vans or heavy vehicles.

[0004] Since fuel economies and the need to protect the environment havebecome priorities, it has proved necessary to produce tires havingreduced rolling resistance. This has been made possible in particulardue to the discovery of new rubber compositions reinforced with specificinorganic fillers referred to as “reinforcing” fillers, which arecapable of rivalling conventional carbon black from the reinforcingpoint of view, and furthermore offering these compositions a lowhysteresis, which is synonymous with lesser rolling resistance for thetires comprising them.

[0005] Such rubber compositions, comprising reinforcing inorganicfillers of the siliceous or aluminous type, have for example beendescribed in patents or patent applications EP-A-0 501 227 (or U.S. Pat.No. 5,227,425), EP-A-0 735 088 (or U.S. Pat. No. 5,852,099), EP-A-0 810258 (or U.S. Pat. No. 5,900,449), EP-A-0 881 252, WO99/02590,WO99/02601, WO99/02602, WO99/28376, WO00/05300 and WO00/05301.

[0006] Mention will be made in particular of documents EP-A-0 501 227,EP-A-0 735 088 or EP-A-0 881 252, which disclose diene rubbercompositions reinforced with precipitated silicas of the highlydispersible type, such compositions making it possible to manufacturetreads having a significantly improved rolling resistance, withoutadversely affecting the other properties, in particular those of grip,endurance and wear resistance. Such compositions having such anexcellent compromise of contradictory properties are also described inapplications EP-A-0 810 258 and WO99/28376, with specific aluminousfillers (alumina or aluminium oxide-hydroxides) of high dispersibilityas reinforcing inorganic fillers.

[0007] Ideally, a tire tread must meet a large number of technicaldemands, which are frequently contradictory, including: high wearresistance, low rolling resistance, very good grip both on dry groundand on wet, snow-covered or icy ground, while offering the tire a verygood level of road behaviour (“handling”) on an automobile, inparticular high drift thrust (“cornering”).

[0008] To improve the road behaviour, it is known that greater rigidityof the tread is desirable, this stiffening of the tread possibly beingobtained for example by increasing the amount of reinforcing filler orby incorporating certain reinforcing resins into the rubber compositionsconstituting these treads.

[0009] However, such stiffening of the tread, at the very least for itssurface part which is in contact with the ground during running of thetire, in known manner impairs, most frequently in crippling manner, theproperties of grip on wet, snow-covered or icy ground.

[0010] To meet these two contradictory demands, namely road behaviourand grip, it has essentially been proposed hitherto to use compositetreads (i.e. hybrid treads), formed by two radially superposed layers(“cap-base structure”) of different rigidity, formed of two rubbercompositions of different formulations: the radially outer layer, incontact with the road, is formed of the most flexible composition, inorder to meet the grip requirements; the radially inner layer is formedof the most rigid composition, in order to meet the road-behaviourrequirements.

[0011] Such a solution however has numerous disadvantages:

[0012] first of all, the manufacturing of a composite tread is bydefinition more complex and therefore more costly than that of aconventional tread, and requires in particular the use of complexcoextrusion machines;

[0013] during manufacturing, after cutting out the tread to the correctdimensions once it has emerged from the extruder, it is furthermorenecessary to manage discarding of material of different natures, whichfurther substantially increases the production costs;

[0014] finally, and this is not the least of the disadvantages, once theradially outer (flexible) part of the tread has become worn, it is theinitially inner part of the tread which comes into contact with theroad: then, of course, one has the disadvantages of an excessively rigidtread, with unsatisfactory performance from the point of view of thetechnical compromise initially intended.

[0015] Now, the Applicants have discovered during their research that aspecific rubber composition, based on a high amount of reinforcinginorganic filler and a methylene acceptor/donor system, makes itpossible, owing to an unexpected “auto-accommodation” phenomenon, toobtain a tread having a true rigidity gradient, radially increasing fromthe surface towards the inside of the tread. This rigidity gradient isnot only obtained simply and economically, but also durably, thus makingit possible to keep the compromise between grip and road behaviour ofthe tires at a very high level, throughout the life of the latter.

[0016] Consequently, a first subject of the invention relates to a tiretread formed, at least in part, of a rubber composition based on atleast (phr: parts by weight per hundred parts of elastomer):

[0017] (i) a diene elastomer;

[0018] (ii) more than 50 phr of an inorganic filler as reinforcingfiller;

[0019] (iii) between 2 and 15 phr of an (inorganic filler/dieneelastomer) coupling agent;

[0020] (iv) between 1 and 10 phr of a methylene acceptor, and

[0021] (v) between 0.5 and 5 phr of a methylene donor.

[0022] The subject of the invention is also the use of such a tread forthe manufacturing of new tires or the recapping of worn tires.

[0023] The tread according to the invention is particularly suited totires intended to be fitted on passenger vehicles, 4×4 vehicles (having4 driving wheels), motorcycles, vans and heavy vehicles.

[0024] The subject of the invention is also these tires themselves, whenthey comprise a tread according to the invention. It relates inparticular to tires of “winter” type intended for snow-covered or icyroads.

[0025] Another subject of the invention is a process for preparing asulphur-vulcanizable tire tread, liable to have, after mechanicalrunning-in of the tire for which it is intended, a rigidity gradientwhich increases radially from the surface towards the inside of thetread, this process being characterized in that it comprises thefollowing steps:

[0026] incorporating in a diene elastomer, in a mixer:

[0027] more than 50 phr of an inorganic filler as reinforcing filler;

[0028] between 2 and 15 phr of an (inorganic filler/diene elastomer)coupling agent;

[0029] between 1 and 10 phr of a methylene acceptor,

[0030] by thermomechanically kneading the entire mixture, in one or morestages, until a maximum temperature of between 130° C. and 200° C. isreached;

[0031] cooling the entire mixture to a temperature of less than 100° C.;

[0032] then incorporating:

[0033] between 0.5 and 5 phr of a methylene donor,

[0034] a vulcanization system;

[0035] kneading the entire mixture until a maximum temperature less than120° C. is reached;

[0036] extruding or calendering the rubber composition thus obtained, inthe form of a tire tread.

[0037] The invention and its advantages will be readily understood inthe light of the description and examples of embodiment which follow,and of the sole FIGURE relating to these examples, which shows, fordifferent treads, whether or not in accordance with the invention,curves of the variation of rigidity (modulus M10) as a function of thedepth (“e” in mm) in these treads.

I. MEASUREMENTS AND TESTS USED

[0038] The treads and rubber compositions constituting these treads arecharacterized, before and after curing, as indicated hereafter.

[0039] I-1. Mooney Plasticity

[0040] An oscillating consistometer such as described in French StandardNF T 43-005 (November 1980) is used. The Mooney plasticity is measuredin accordance with the following principle: the raw composition (i.e.before curing) is moulded in a cylindrical enclosure heated to 100° C.After one minute's preheating, the rotor turns within the test piece at2 rpm, and the torque used for maintaining this movement is measuredafter four minutes' rotation. The Mooney plasticity (ML 1+4) isexpressed in “Mooney units” (MU, with 1 MU=0.83 Newton meter).

[0041] I-2. Scorching Time

[0042] The measurements are effected at 130° C., in accordance withFrench Standard NF T 43-005. The evolution of the consistometric indexas a function of time makes it possible to determine the scorching timefor the rubber compositions, assessed in accordance with the abovestandard by the parameter T5 (case of a large rotor), expressed inminutes, and defined as being the time necessary to obtain an increasein the consistometric index (expressed in MU) of 5 units above theminimum value measured for this index.

[0043] I-3. Tensile Tests

[0044] These tensile tests make it possible to determine the elasticitystresses and the properties at break. Unless indicated otherwise, theyare effected in accordance with French Standard NF T 46-002 of September1988. The nominal secant moduli (or apparent stresses, in MPa) at 10%elongation (M10), 100% elongation (M100) and 300% elongation (M300) aremeasured in a second elongation (i.e. after a cycle of accommodation tothe amount of extension provided for the measurement itself).

[0045] The nominal secant modulus is also measured at 10% elongation,after an accommodation of 15% (i.e. an extension to 15% followed byrelaxation to 0%) and not 10% as above for the modulus M10. Thisso-called “accommodated” modulus is referred to as M10_(Ac). Thebreaking stresses (in MPa) and the elongations at break (in %) are alsomeasured. All these tensile measurements are effected under normalconditions of temperature (23±2° C.) and humidity (50±5% relativehumidity), in accordance with French standard NFT 40-101 (December1979).

[0046] I-4. Shore A Hardness

[0047] The Shore A hardness of the compositions after curing is assessedin accordance with ASTM Standard D 2240-86.

[0048] I-5. Dynamic Properties

[0049] The dynamic properties ΔG* and tan(δ)_(max) are measured on aviscoanalyser (Metravib VA4000), in accordance with ASTM StandardD5992-96. The response of a sample of vulcanized composition(cylindrical test piece of a thickness of 4 mm and a section of 400mm²), subjected to an alternating single sinusoidal shearing stress, ata frequency of 10 Hz, under normal conditions of temperature (23° C.) inaccordance with Standard ASTM D 1349-99, is recorded. Scanning waseffected at an amplitude of deformation of 0.1 to 50% (outward cycle),then of 50% to 1% (return cycle). The results used are the complexdynamic shear modulus (G*) and the loss factor tan(δ). For the returncycle, the maximum value of tan(δ) which is observed is indicated(tan(δ)_(max)), and the deviation in the complex modulus (ΔG*) betweenthe values at 0.15% and 50% deformation (Payne effect).

[0050] I-6. Tests on Tires

[0051] A) Shore A Hardness:

[0052] It is measured on the outer surface of the tread, the one incontact with the ground, in accordance with the aforementioned standardASTM D 2240-86.

[0053] B) Drift Thrust:

[0054] Each tire tested is mounted on a wheel of suitable dimension andinflated to 2.2 bar. It is made to run at a constant speed of 80 km/h ona suitable automatic machine (“flat-belt” type test machine sold byMTS). The load “Z” is varied, at a drift angle of 1 degree, and therigidity or drift thrust “D” (corrected for the thrust at zero drift) ismeasured in known manner, by recording the transverse force on the wheelas a function of this load Z using sensors. The drift thrust indicatedin the tables is the gradient at the origin of the curve D(Z).

[0055] C) Accommodation or Mechanical Running-in:

[0056] Some of the characteristics of the tires tested may be measuredboth on new tires (that is to say in the initial state, before anyrunning) and on tires which have undergone mechanical “accommodation” oftheir treads.

[0057] “Mechanical accommodation” is to be understood to mean heresimple running-in of the tire by means of which its tread is placed incontact with the ground during running, that is to say in workingconditions, for several tens of seconds or several minutes at the most.This running-in operation may be carried out on an automatic runningmachine or directly on an automobile, and effected in various ways, forexample by simply running in a straight line of several tens or hundredsof meters, by longitudinal braking or alternatively by drifting of thetire (bends), the important thing being to start making the tread “work”under normal conditions of use.

[0058] For the requirements of the tire tests which follow, themechanical accommodation is achieved, unless indicated otherwise in therest of the text, by what is called “standard” running-in consisting ofsimple running in a straight line over a length of 400 meters at a speedof 60 km/h, on a given automobile, without drifting or cambering imposedon the tire, followed by moderate longitudinal braking (braking distancefrom 30 to 40 meters) to stop the vehicle. This “standard running-in” isfurthermore effected under normal conditions of pressure (thoserecommended by the manufacturer of the vehicle used) and load (1 persononly on board the vehicle).

[0059] D) Braking on Damp Ground:

[0060] The tires are mounted on an automobile fitted with an ABS brakingsystem and the distance necessary to go from 40 km/h to 10 km/h uponsudden braking on wetted ground (asphalt concrete) is measured. A valuegreater than that of the control, arbitrarily set to 100, indicates animproved result, that is to say a shorter braking distance.

[0061] E) Grip on Damp Ground:

[0062] To assess the grip performance on damp ground, the behaviour ofthe tires mounted on a given automobile travelling round a circuitcomprising numerous bends and wetted so as to keep the ground damp,under limit speed conditions is analysed.

[0063] On one hand the minimum time necessary to cover the entirecircuit is measured; a value less than that of the control, arbitrarilyset to 100, indicates an improved result, that is to say a shortertravelling time.

[0064] The professional driver of the vehicle, on the other hand,assigns a subjective overall mark for road behaviour of the vehicle—andtherefore of the tires—on this wetted circuit comprising bends; a markgreater than that of the control, arbitrarily set to 100, indicatesimproved overall behaviour.

II. CONDITIONS OF CARRYING OUT THE INVENTION

[0065] The treads according to the invention are formed, at least inpart, of a rubber composition based on at least: (i) a (at least one)diene elastomer; (ii) a minimum quantity (more than 50 phr) of a (atleast one) inorganic filler as reinforcing filler; (iii) a (at leastone) coupling agent (between 2 and 15 phr) providing the bond betweenthe reinforcing inorganic filler and this diene elastomer; (iv) a (atleast one) acceptor (between 1 and 10 phr) and (v) a (at least one)methylene donor (between 0.5 and 5 phr).

[0066] Of course, the expression composition “based on” is to beunderstood to mean a composition comprising the mix and/or the productof reaction in situ of the various constituents used, some of these baseconstituents (for example, the coupling agent, the methylene acceptorand donor) being liable to, or intended to, react together, at least inpart, during the different phases of manufacturing of the treads, inparticular during the vulcanization (curing) thereof.

[0067] II-1. Diene Elastomer

[0068] “Diene” elastomer or rubber is understood to mean, generally, anelastomer resulting at least in part (i.e., a homopolymer or acopolymer) from diene monomers (monomers bearing two doublecarbon-carbon bonds, whether conjugated or not). “Essentiallyunsaturated” diene elastomer is understood here to mean a dieneelastomer resulting at least in part from conjugated diene monomers,having a content of members or units of diene origin (conjugated dienes)which is greater than 15% (mol %). Thus, for example, diene elastomerssuch as butyl rubbers or copolymers of dienes and of alpha-olefins ofthe EPDM type do not fall within this definition, and may on thecontrary be described as “essentially saturated” diene elastomers (lowor very low content of units of diene origin which is always less than15%). Within the category of “essentially unsaturated” diene elastomers,“highly unsaturated” diene elastomer is understood to mean in particulara diene elastomer having a content of units of diene origin (conjugateddienes) which is greater than 50%.

[0069] These general definitions being given, the person skilled in theart of tires will understand that the present invention is used firstand foremost with highly unsaturated diene elastomers, in particularwith:

[0070] (a)—any homopolymer obtained by polymerisation of a conjugateddiene monomer having 4 to 12 carbon atoms;

[0071] (b)—any copolymer obtained by copolymerization of one or moreconjugated dienes with each other or with one or more vinyl-aromaticcompounds having 8 to 20 carbon atoms.

[0072] Suitable conjugated dienes are, in particular, 1,3-butadiene,2-methyl-1,3-butadiene, 2,3-di(C₁-C₅ alkyl)-1,3-butadienes such as, forinstance, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, anaryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. Suitablevinyl-aromatic compounds are, for example, styrene, ortho-, meta- andpara-methylstyrene, the commercial mixture “vinyltoluene”,para-tert.-butylstyrene, methoxystyrenes, chlorostyrenes,vinylmesitylene, divinylbenzene and vinylnaphthalene.

[0073] The copolymers may contain between 99% and 20% by weight of dieneunits and between 1% and 80% by weight of vinyl-aromatic units. Theelastomers may have any microstructure, which is a function of thepolymerisation conditions used, in particular of the presence or absenceof a modifying and/or randomising agent and the quantities of modifyingand/or randomising agent used. The elastomers may for example be block,statistical, sequential or microsequential elastomers, and may beprepared in dispersion or in solution; they may be coupled and/orstarred or alternatively functionalized with a coupling and/or starringor functionalizing agent.

[0074] Preferred are polybutadienes, and in particular those having acontent of 1,2-units between 4% and 80%, or those having a content ofcis-1,4 [bonds] of more than 80%, polyisoprenes, butadiene-styrenecopolymers, and in particular those having a styrene content of between5% and 50% by weight and, more particularly, between 20% and 40%, acontent of 1,2-bonds of the butadiene fraction of between 4% and 65%,and a content of trans-1,4 bonds of between 20% and 80%,butadiene-isoprene copolymers and in particular those having an isoprenecontent of between 5% and 90% by weight and a glass transitiontemperature (“Tg”-measured in accordance with ASTM Standard D3418-82) ofbetween −40° C. and −80° C., isoprene-styrene copolymers and inparticular those having a styrene content of between 5% and 50% byweight and a Tg of between −25° C. and −50° C. In the case ofbutadiene-styrene-isoprene copolymers, those which are suitable are inparticular those having a styrene content of between 5% and 50% byweight and, more particularly, between 10% and 40%, an isoprene contentof between 15% and 60% by weight, and more particularly between 20% and50%, a butadiene content of between 5% and 50% by weight, and moreparticularly between 20% and 40%, a content of 1,2-units of thebutadiene fraction of between 4% and 85%, a content of trans-1,4 unitsof the butadiene fraction of between 6% and 80%, a content of 1,2- plus3,4-units of the isoprene fraction of between 5% and 70%, and a contentof trans-1,4 units of the isoprene fraction of between 10% and 50%, andmore generally any butadiene-styrene-isoprene copolymer having a Tg ofbetween −20° C. and −70° C.

[0075] In summary, particularly preferably, the diene elastomer of thecomposition according to the invention is selected from the group ofhighly unsaturated diene clastomers which consists of polybutadienes(BR), synthetic polyisoprenes (IR), natural rubber (NR), butadienecopolymers, isoprene copolymers and mixtures of these elastomers.

[0076] Such copolymers are more preferably selected from the group whichconsists of butadiene-styrene copolymers (SBR), butadiene-isoprenecopolymers (BIR), isoprene-styrene copolymers (SIR),isoprene-butadiene-styrene copolymers (SBIR) and mixtures of suchcopolymers.

[0077] The tread according to the invention is preferably intended for apassenger-car tire. In such a case, the diene elastomer is preferably anSBR copolymer, in particular an SBR prepared in solution, preferablyused in a mixture with a polybutadiene; more preferably, the SBR has acontent of styrene of between 20% and 30% by weight, a content of vinylbonds of the butadiene fraction of between 15% and 65%, a content oftrans-1,4 bonds of between 15% and 75% and a Tg of between −20° C. and−55° C., and the polybutadiene has more than 90% cis-1,4 bonds.

[0078] The compositions of the treads of the invention may contain asingle diene elastomer or a mixture of several diene elastomers, thediene elastomer(s) possibly being used in association with any type ofsynthetic elastomer other than a diene one, or even with polymers otherthan elastomers, for example thermoplastic polymers.

[0079] II-2. Reinforcing Inorganic Filler

[0080] “Reinforcing inorganic filler” is to be understood here to meanany inorganic or mineral filler, whatever its colour and its origin(natural or synthetic), also referred to as “white” filler or sometimes“clear” filler in contrast to carbon black, this inorganic filler beingcapable, on its own, without any other means than an intermediatecoupling agent, of reinforcing a rubber composition intended for themanufacturing of a tire tread, in other words which is capable ofreplacing a conventional tire-grade carbon black (for treads) in itsreinforcement function.

[0081] Preferably, the reinforcing inorganic filler is a filler of thesiliceous or aluminous type, or a mixture of these two types of fillers.

[0082] The silica (SiO₂) used may be any reinforcing silica known to theperson skilled in the art, in particular any precipitated or pyrogenicsilica having a BET surface area and a specific CTAB surface area bothof which are less than 450 m²/g, preferably from 30 to 400 m²/g. Highlydispersible precipitated silicas (referred to as “HD”) are preferred, inparticular when the invention is used for the manufacturing of tireshaving a low rolling resistance; “highly dispersible silica” isunderstood in known manner to mean any silica having a substantialability to disagglomerate and to disperse in an elastomeric matrix,which can be observed in known manner by electron or optical microscopyon thin sections. As non-limitative examples of such preferred highlydispersible silicas, mention may be made of the silicas BV3380 andUltrasil 7000 from Degussa, the silicas Zeosil 1165 MP and 1115 MP fromRhodia, the silica Hi-Sil 2000 from PPG, the silicas Zeopol 8715 or 8745from Huber, and treated precipitated silicas such as, for example, thealuminium-“doped” silicas described in application EP-A-0 735 088.

[0083] The reinforcing alumina (Al₂O₃) preferably used is a highlydispersible alumina having a BET surface area from 30 to 400 m²/g, morepreferably between 60 and 250 m²/g, an average particle size at mostequal to 500 nm, more preferably at most equal to 200 nm, as describedin the aforementioned application EP-A-0 810 258. Non-limitativeexamples of such reinforcing aluminas are in particular the aluminasA125 or CR125 (from Baikowski), APA-100RDX (from Condea), Aluminoxid C(from Degussa) or AKP-G015 (Sumitomo Chemicals). The invention can alsobe implemented by using as reinforcing inorganic filler the specificaluminium (oxide-)hydroxides such as described in the aforementionedapplication WO99/28376.

[0084] The physical state in which the reinforcing inorganic filler ispresent is immaterial, whether it be in the form of a powder,microbeads, granules, balls or any other densified form.

[0085] Of course, “reinforcing inorganic filler” is also understood tomean mixtures of different reinforcing inorganic fillers, in particularof highly dispersible siliceous and/or aluminous fillers such asdescribed above.

[0086] When the treads of the invention are intended for tires of lowrolling resistance, the reinforcing inorganic filler used, in particularif it is silica, preferably has a BET surface area of between 60 and 250m²/g, more preferably between 80 and 230 m²/g.

[0087] The inorganic filler used as reinforcing filler must be presentin an amount greater than 50 phr, which is one of the essentialcharacteristics of the invention. This reinforcing inorganic filler mayconstitute all or the majority of the total reinforcing filler, in thislatter case associated for example with a minority quantity of carbonblack.

[0088] Preferably, the amount of reinforcing inorganic filler is between60 and 120 phr, more preferably still within a range from 70 to 100 phrapproximately, in particular when the tread is intended for apassenger-car tire. The person skilled in the art will readilyunderstand that the optimum will be different according to the nature ofthe reinforcing inorganic filler used and according to the type of tirein question, for example a tire for a motorcycle, passenger vehicle oralternatively for a utility vehicle such as a van or a heavy vehicle.

[0089] Preferably, in the tread according to the invention, thereinforcing inorganic filler constitutes more than 80% by weight of thetotal reinforcing filler, more preferably more than 90% by weight (oreven all) of this total reinforcing filler.

[0090] However, without significantly affecting the technical effectdesired, a small quantity of carbon black, preferably less than 20%,more preferably still less than 10% by weight relative to the quantityof total reinforcing filler, may be used.

[0091] The carbon black, if it is used, is preferably present in anamount of between 2 and 15 phr, more preferably between 4 and 12 phr. Itcan be used in particular as a simple black pigmentation agent, oralternatively to protect the tread from different sources of atmosphericageing such as ozone, oxidation or UV radiation. On the other hand, itis known that certain rubber-making additives, in particular certaincoupling agents, are available in a form supported by carbon black, theuse of such additives therefore involving the incorporation, in a smallproportion, of carbon black. Suitable carbon blacks are any carbonblacks, in particular the blacks of the type HAF, ISAF and SAF, whichare conventionally used in tires, and particularly in treads for thesetires; as non-limitative examples of such blacks, mention may be made ofthe blacks N115, N134, N234, N339, N347 and N375.

[0092] In the present specification, the “BET” specific surface area isdetermined in known manner, in accordance with the method of Brunauer,Emmett and Teller described in “The Journal of the American ChemicalSociety”, vol. 60, page 309, February 1938, and corresponding to FrenchStandard NFT 45-007 (November 1987); the CTAB specific surface area isthe external surface area determined in accordance with the sameStandard NF T 45-007.

[0093] Finally, as filler equivalent to such a reinforcing inorganicfiller, there could be used a reinforcing filler of organic type, inparticular a carbon black, covered at least in part with an inorganiclayer (for example, a layer of silica), which for its part requires theuse of a coupling agent to provide the connection to the elastomer.

[0094] II-3. Coupling Agent

[0095] In known manner, in the presence of a reinforcing inorganicfiller, it is necessary to use a coupling agent, also referred to asbonding agent, the function of which is to provide the connection orbond between the surface of the particles of this inorganic filler andthe diene elastomer, while facilitating the dispersion of this inorganicfiller within the elastomeric matrix during the thermomechanicalkneading.

[0096] It will be recalled that (inorganic filler/elastomer) “couplingagent” is to be understood to mean an agent capable of establishing asufficient chemical and/or physical connection between the inorganicfiller and the elastomer; such a coupling agent, which is consequentlyat least bifunctional, has, for example, the simplified general formula“Y-T-X”, in which:

[0097] Y represents a functional group (function “Y”) which is capableof bonding physically and/or chemically with the inorganic filler, sucha bond being able to be established, for example, between a silicon atomof the coupling agent and the surface hydroxyl (OH) groups of theinorganic filler (for example, surface silanols in the case of silica);

[0098] X represents a functional group (function “X”) which is capableof bonding physically and/or chemically with the diene elastomer, forexample by means of a sulphur atom;

[0099] T represents a divalent organic group making it possible to linkY and X.

[0100] The coupling agents must particularly not be confused with simpleagents for covering the inorganic filler which, in known manner, maycomprise the function Y which is active with respect to the inorganicfiller but are devoid of the function X which is active with respect tothe elastomer.

[0101] (Silica/diene elastomer) coupling agents, of variableeffectiveness, have been described in a very large number of documentsand are well-known to the person skilled in the art. Any known couplingagent likely to ensure, in the diene rubber compositions which can beused for the manufacturing of tire treads, the bonding between areinforcing inorganic filler such as silica and a diene elastomer, inparticular organosilanes or polyfunctional polyorganosiloxanes bearingthe functions X and Y mentioned above, may be used.

[0102] In particular polysulphurized silanes, which are referred to as“symmetrical” or “asymmetrical” depending on their specific structure,are used, such as those described for example in the patents or patentapplications FR 2 149 339, FR 2 206 330, U.S. Pat. No. 3,842,111, U.S.Pat. No. 3,873,489, U.S. Pat. No. 3,978,103, U.S. Pat. No. 3,997,581,U.S. Pat. No. 4,002,594, U.S. Pat. No. 4,072,701, U.S. Pat. No.4,129,585, U.S. Pat. No. 5,580,919, U.S. Pat. No. 5,583,245, U.S. Pat.No. 5,650,457, U.S. Pat. No. 5,663,358, U.S. Pat. No. 5,663,395, U.S.Pat. No. 5,663,396, U.S. Pat. No. 5,674,932, U.S. Pat. No. 5,675,014,U.S. Pat. No. 5,684,171, U.S. Pat. No. 5,684,172, U.S. Pat. No.5,696,197, U.S. Pat. No. 5,708,053, U.S. Pat. No. 5,892,085, EP 1 043357.

[0103] Particularly suitable for implementing the invention, without thedefinition below being limitative, are so-called “symmetrical”polysulphurized silanes which satisfy the following general formula (I):

Z-A-S_(n)-A-Z,  (I)

[0104] in which:

[0105] n is an integer from 2 to 8 (preferably from 2 to 5);

[0106] A is a divalent hydrocarbon radical (preferably C₁-C₁₈ alkylenegroups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀ alkylenes,notably C₁-C₄ alkylenes, in particular propylene);

[0107] Z corresponds to one of the formulae below:

[0108] in which:

[0109] the radicals R¹, which may or may not be substituted, and may beidentical or different, represent a C₁-C₁₈ alkyl group, a C₅-C₁₈cycloalkyl group or a C₆-C₁₈ aryl group, (preferably C₁-C₆ alkyl groups,cyclohexyl or phenyl, in particular C₁-C₄ alkyl groups, moreparticularly methyl and/or ethyl).

[0110] the radicals R², which may or may not be substituted, and may beidentical or different, represent a C₁-C₁₈ alkoxyl group or a C₅-C₁₈cycloalkoxyl group (preferably a group selected from among C₁-C₈alkoxyls and C₅-C₈ cycloalkoxyls, more preferably still a group selectedfrom among C₁-C₄ alkoxyls, in particular methoxyl and/or ethoxyl).

[0111] In the case of a mixture of polysulphurized alkoxysilanes inaccordance with Formula (I) above, in particular conventional,commercially available, mixes, the average value of the “n”s is afractional number, preferably between 2 and 5, more preferably close to4. However, the invention may also be implemented advantageously forexample with disulphurized alkoxysilanes (n=2).

[0112] As examples of polysulphurized silanes, mention will be made moreparticularly of the polysulphides (in particular disulphides,trisulphides or tetrasulphides) ofbis-((C₁-C₄)alkoxyl-(C₁-C₄)alkylsilyl(C₁-C₄)alkyl), such as for examplethe polysulphides of bis(3-trimethoxysilylpropyl) or ofbis(3-triethoxysilylpropyl). Of these compounds, in particularbis(3-triethoxysilylpropyl) tetrasulphide, abbreviated TESPT, of theformula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, or bis(triethoxysilylpropyl) disulphide,abbreviated TESPD, of the formula [(C₂H₅O)₃Si(CH₂)₃S]₂, are used.

[0113] TESPD is sold, for example, by Degussa under the name Si75 (inthe form of a mixture of disulphide—75% by weight—and of polysulphides),or alternatively by Witco under the name Silquest A1589. TESPT is sold,for example, by Degussa under the name Si69 (or X50S when it issupported to 50% by weight on carbon black), or alternatively by OsiSpecialties under the name Silquest A1289 (in both cases, a commercialmixture of polysulphides having an average value of n which is close to4).

[0114] By way of examples of coupling agents other than theaforementioned polysulphurized alkoxysilanes, mention will be made inparticular of the bifunctional polyorganosiloxanes such as described forexample in the aforementioned application WO99/02602.

[0115] In the treads according to the invention, the content of couplingagent is preferably between 4 and 12 phr, more preferably between 3 and8 phr. However, it is generally desirable to use as little as possiblethereof.

[0116] Relative to the weight of reinforcing inorganic filler, theamount of coupling agent preferably represents between 0.5 and 15% byweight relative to the quantity of reinforcing inorganic filler. Morepreferably, in the case of treads for tires for passenger vehicles, thecoupling agent is present in an amount less than 12%, or even less than10% by weight relative to this quantity of reinforcing inorganic filler.

[0117] The polysulphurized alkoxysilane could first be grafted (via thefunction “X”) on to the diene elastomer of the composition of theinvention, the elastomer thus functionalized or “precoupled” thencomprising the free function “Y” for the reinforcing inorganic filler.The polysulphurized alkoxysilane could also be grafted beforehand (viathe function “Y”) on the reinforcing inorganic filler, the thus“precoupled” filler then being able to be bonded to the diene elastomerby means of the free function “X”. However, it is preferred, inparticular for reasons of better processing of the compositions in theuncured state, to use the coupling agent, either grafted onto thereinforcing inorganic filler, or in the free (i.e. non-grafted) state.

[0118] There may possibly be associated with the coupling agent anappropriate “coupling activator”, that is to say, a body (singlecompound or association of compounds) which, when mixed with thiscoupling agent, increases the effectiveness of the latter. Couplingactivators for polysulphurized alkoxysilanes have for example beendescribed in the aforementioned international applications WO00/05300and WO00/05301, consisting of the association of a substitutedguanidine, in particular N,N′-diphenylguanidine (abbreviated to “DPG”),with an enamine or a zinc dithiophosphate. The presence of thesecoupling activators will make it possible, for example, to keep theamount of coupling agent at a preferred level of less than 10%, or evenless than 8% by weight relative to the quantity of reinforcing inorganicfiller, or alternatively to reduce the amount of reinforcing inorganicfiller owing to the improved coupling with the diene elastomer.

[0119] II-4. Methylene Acceptor/Donor System (So-called “M.A.D.” System)

[0120] The terms “methylene acceptor” and “methylene donor” arewell-known to the person skilled in the art and are widely used todesignate compounds liable to react together to generate athree-dimensional reinforcing resin by condensation.

[0121] The rubber compositions of the treads of the invention contain,in combination, at least one methylene acceptor associated with at leastone methylene donor, which are intended to form in situ, after curing(vulcanization) of the tread, a three-dimensional resin lattice which issuperposed and interpenetrates with the (inorganic filler/elastomer)lattice on one hand, and with the (elastomer/sulphur) lattice on theother hand (if the cross-linking agent is sulphur).

[0122] Methylene acceptors, in particular novolac resins, whether or notassociated with a methylene donor, had admittedly already been describedin rubber compositions, in particular those intended for tires or treadsfor tires, for applications as varied as grip or reinforcement:reference will be made, for example, to documents EP-A-0 967 244, U.S.Pat. No. 6,028,137, U.S. Pat. No. 6,015,851, U.S. Pat. No. 5,990,210,U.S. Pat. No. 5,872,167, U.S. Pat. No. 5,859,115 or EP-A-0 736 399, U.S.Pat. No. 5,840,113 or EP-A-0 875 532, U.S. Pat. No. 5,405,897, U.S. Pat.No. 5,049,418 and U.S. Pat. No. 4,837,266.

[0123] However, as far as the Applicant is aware, no document of theprior art describes the use in a tire tread, in the proportions setforth here, of such an M.A.D. system in combination with a so highamount (more than 50 phr, preferably more than 60 phr) of a reinforcinginorganic filler such as silica.

[0124] A) Methylene acceptor

[0125] In known manner, the term “methylene acceptor” designates thereactant with which the methylene donor compound reacts by formation ofmethylene bridges (—CH2—), upon the curing of the composition, thusresulting in the formation in situ of the three-dimensional resinlattice.

[0126] The methylene acceptor must be able to disperse perfectly in therubber matrix, at the same time as the reinforcing inorganic filler andits coupling agent.

[0127] Particularly suitable are phenols, the generic name forhydroxylated derivatives of arenes and the equivalent compounds; such adefinition covers in particular monophenols, for example phenyl properor hydroxybenzene, bisphenols, polyphenols (polyhydroxyarenes),substituted phenols such as alkylphenols or aralkylphenols, for examplebisphenols, diphenylolpropane, diphenylolmethane, naphthols, cresol,t-butylphenol, octylphenol, nonylphenol, xylenol, resorcinol oranalogous products.

[0128] Preferably phenolic resins referred to as “novolac resins”, alsocalled phenol-aldehyde precondensates, resulting from theprecondensation of phenolic compounds and aldehydes, in particularformaldehyde, are used. In known manner, these novolac resins (alsoreferred to as “two-step resins”) are thermoplastic and require the useof a curing agent (methylene donor) to be cross-linked, unlike, forexample, resols which are thermohardening; they have sufficientplasticity not to interfere with the processing of the rubbercomposition. After cross-linking by the methylene donor (they may thenbe referred to as “thermohardening” novolac resins), they arecharacterized in particular by a tighter three-dimensional lattice thanthat of the resols.

[0129] The quantity of methylene acceptor must be between 1 and 10 phr;below the minimum indicated, the technical effect desired is inadequate,whereas beyond the maximum indicated there are the risks of excessivestiffening and excessive compromising of the hysteresis. For all thesereasons, a quantity of between 2 and 8 phr is more preferably selected,amounts lying within a range from 3 to 6 phr being particularlyadvantageous in the case of treads for passenger-car tires.

[0130] Finally, the quantity of methylene acceptor, in the rangesindicated above, is advantageously adjusted so as to represent between2% and 15%, more preferably between 4% and 10%, by weight relative tothe quantity of reinforcing inorganic filler.

[0131] B) Methylene Donor

[0132] A curing agent, capable of cross-linking or hardening themethylene acceptor previously described, also commonly referred to as“methylene donor”, must be associated with this acceptor. Thecross-linking of the resin is then caused during the curing of therubber matrix, by formation of (—CH₂—) bridges.

[0133] Preferably, the methylene donor is selected from the groupconsisting of hexamethylenetetramine (abbreviated to HMT),hexamethoxymethylmelamine (abbreviated to HMMM or H3M),hexaethoxymethylmelamine, formaldehyde polymers such as p-formaldehyde,N-methylol melamine derivatives, or mixtures of these compounds. Morepreferably, a methylene donor selected from among HMT, H3M or a mixtureof these compounds is used.

[0134] The quantity of methylene donor must be between 0.5 and 5 phr;below the minimum indicated, the technical effect desired has provedinadequate, whereas beyond the maximum indicated there is the risk ofimpairing the processing in the uncured state of the compositions (forexample, problem of solubility of the HMT) or the vulcanization (sloweddown in the presence of H3M). For all these reasons, a quantity ofbetween 0.5 and 3.5 phr is more preferably selected, amounts lyingwithin a range from 1 to 3 phr being particularly advantageous in thecase of treads for passenger-car tires.

[0135] Finally, the quantity of methylene donor, in the ranges indicatedabove, is advantageously adjusted so as to represent between 10% and80%, more preferably within a range from 40% to 60%, by weight relativeto the quantity of methylene acceptor.

[0136] II-6. Various Additives

[0137] Of course, the rubber compositions of the treads according to theinvention also comprise all or part of the additives usually used insulphur-cross-linkable diene rubber compositions intended for themanufacturing of treads, such as, for example, plasticizers, pigments,protective agents of the type antioxidants, antiozonants, across-linking system based either on sulphur or on sulphur and/orperoxide and/or bismaleimide donors, vulcanization accelerators,vulcanization activators, extender oils, etc. There may also beassociated with the reinforcing inorganic filler, if necessary, aconventional non-reinforcing white filler, such as for example particlesof clay, bentonite, talc, chalk, kaolin or titanium oxides.

[0138] The rubber compositions of the treads of the invention may alsocontain, in addition to the coupling agents, agents for covering thereinforcing inorganic filler (comprising for example the single functionY), or more generally processing aids liable, in known manner, owing toan improvement in the dispersion of the inorganic filler in the rubbermatrix and to a reduction in the viscosity of the compositions, toimprove their ability to be worked in the uncured state, these agentsbeing, for example, alkylalkoxysilanes, (in particularalkyltriethoxysilanes), polyols, polyethers (for example polyethyleneglycols), primary, secondary or tertiary amines, hydroxylated orhydrolyzable polyorganosiloxanes, for exampleα,ω)-dihydroxypolyorganosiloxanes (in particularα,ω-dihydroxy-polydimethylsiloxanes).

[0139] II-7. Manufacturing of the Treads

[0140] The rubber compositions of the treads of the invention aremanufactured in suitable mixers, using two successive preparation phasesin accordance with a general process well-known to the person skilled inthe art (see for example documents EP-A-0 501 227 or WO00/05300mentioned above): a first phase of thermomechanical working or kneading(sometimes referred to as “non-productive” phase) at high temperature,up to a maximum temperature of between 130° C. and 200° C., preferablybetween 145° C. and 185° C., followed by a second phase of mechanicalworking (sometimes referred to as “productive” phase) at lowertemperature, typically less than 120° C., for example between 60° C. and100° C., during which finishing phase the cross-linking or vulcanizationsystem is incorporated.

[0141] The process according to the invention, for preparing asulphur-vulcanizable tire tread, liable to have, after mechanicalrunning-in of the tire for which it is intended, a rigidity gradientradially increasing from the surface towards the inside of the tread,comprises the following steps:

[0142] incorporating in a diene elastomer, in a mixer:

[0143] more than 50 phr of an inorganic filler as reinforcing filler;

[0144] between 2 and 15 phr of an (inorganic filler/diene elastomer)coupling agent;

[0145] between 1 and 10 phr of a methylene acceptor,

[0146] by thermomechanically kneading the entire mixture, in one or morestages, until a maximum temperature of between 130° C. and 200° C. isreached;

[0147] cooling the entire mixture to a temperature of less than 100° C.;

[0148] then incorporating:

[0149] between 0.5 and 5 phr of a methylene donor,

[0150] a vulcanization system;

[0151] kneading the entire mixture until a maximum temperature of lessthan 120° C.;

[0152] extruding or calendering the rubber composition thus obtained, inthe form of a tire tread.

[0153] According to a preferred embodiment of the invention, all thebase constituents of the compositions of the treads according to theinvention, with the exception of the methylene donor and thevulcanization system, namely the reinforcing inorganic filler, thecoupling agent (and any activator), and the methylene acceptor areincorporated intimately by kneading in the diene elastomer during thefirst, so-called non-productive, phase, that is to say that at leastthese different base constituents are introduced into the mixer and arekneaded thermomechanically, in one or more stages, until the maximumtemperature of between 130° C. and 200° C., preferably between 145° C.and 185° C., is reached.

[0154] More preferably, during this first non-productive phase, themethylene acceptor is incorporated in the mixer later than theelastomer, the reinforcing inorganic filler and its coupling agent, whenthe temperature of the composition in the mixer, during kneading, hasreached a value of between 80° C. and 130° C., more preferably between90° C. and 110° C. In actual fact, better effectiveness of the M.A.D.system was noted for such temperature conditions.

[0155] By way of example, the first (non-productive) phase is effectedin a single thermomechanical step during which all the necessaryconstituents, any additional covering agents or processing agents andvarious other additives, with the exception of the methylene donor andthe vulcanization system, are introduced into a suitable mixer, such asa conventional internal mixer. A second stage of thermomechanicalworking may possibly be added, in this internal mixer, for example afteran intermediate cooling stage (preferably to a temperature of less than100° C.), with the aim of making the compositions undergo complementaryheat treatment, in particular in order to improve the dispersion, in theelastomeric matrix, of the reinforcing inorganic filler, the couplingagent and the methylene acceptor.

[0156] After cooling the mixture thus obtained during the firstnon-productive phase, the methylene donor and the vulcanization systemare then incorporated at low temperature, generally in an external mixersuch as an open mill; the entire composition is then mixed (productivephase) for several minutes, for example between 5 and 15 minutes.

[0157] The vulcanization system proper is preferably based on sulphurand a primary vulcanization accelerator, in particular an accelerator ofthe sulphenamide type. To this vulcanization system there are added,incorporated during the first non-productive phase and/or during theproductive phase, various known secondary accelerators or vulcanizationactivators such as zinc oxide, stearic acid, guanidine derivatives (inparticular diphenylguanidine), etc. The amount of sulphur is preferablybetween 0.5 and 3.0 phr, and the amount of the primary accelerator ispreferably between 0.5 and 5.0 phr.

[0158] The final composition thus obtained is then calendered, forexample in the form of a film or a sheet, in particular forcharacterisation in the laboratory, or alternatively extruded in theform of a rubber profiled element usable directly as a tire tread.

[0159] The vulcanization (or curing) is carried out in known manner at atemperature generally between 130° C. and 200° C., for a sufficient timewhich may vary, for example, between 5 and 90 minutes, depending, inparticular, on the curing temperature, the vulcanization system adoptedand the vulcanization kinetics of the composition in question.

[0160] In the process according to the invention, in accordance with thepreceding information given for the rubber compositions, preferably atleast one, more preferably all, of the following characteristics aresatisfied:

[0161] the quantity of reinforcing inorganic filler is between 60 and120 phr;

[0162] the quantity of coupling agent is between 4 and 12 phr;

[0163] the quantity of methylene acceptor is between 2 and 8 phr;

[0164] the quantity of methylene donor is between 0.5 and 3.5 phr;

[0165] the maximum thermomechanical kneading temperature is between 145°C. and 180° C.;

[0166] the reinforcing inorganic filler is a siliceous or aluminousfiller;

[0167] the quantity of carbon black is between 2 and 15 phr;

[0168] the at least bifunctional coupling agent is an organosilane or apolyorganosiloxane;

[0169] the methylene acceptor is selected from the group consisting ofphenolic resins;

[0170] the methylene donor is selected from the group consisting of HMT,H3M, hexaethoxymethylmelamine, para-formaldehyde polymers, N-methylolderivatives of melamine, or a mixture of these compounds;

[0171] the quantity of methylene acceptor is represents 2% and 15% byweight relative to the weight of reinforcing inorganic filler;

[0172] the quantity of methylene donor represents between 10% and 80% byweight relative to the weight of methylene acceptor.

[0173] the diene elastomer is a butadiene-styrene copolymer (SBR),preferably used in a mixture with a polybutadiene;

[0174] the reinforcing inorganic filler represents more than 80% of thetotal reinforcing filler.

[0175] More preferably, in this process, at least one, even morepreferably all, of the following characteristics are satisfied:

[0176] the quantity of inorganic filler lies within a range from 70 to100 phr;

[0177] the quantity of coupling agent is between 3 and 8 phr;

[0178] the quantity of methylene acceptor lies within a range from 3 to6 phr;

[0179] the quantity of methylene donor lies within a range from 1 to 3phr;

[0180] the reinforcing inorganic filler is silica;

[0181] the quantity of carbon black is between 4 and 12 phr;

[0182] the coupling agent is a bis-(C₁-C₄)alkoxylsilylpropylpolysulphide;

[0183] the methylene acceptor is a novolac phenolic resin;

[0184] the methylene donor is selected from the group consisting of HMT,H3M or a mixture of these compounds;

[0185] the quantity of methylene acceptor is between 4% and 10% byweight relative to the weight of reinforcing inorganic filler;

[0186] the quantity of methylene donor represents from 40% to 60% byweight relative to the weight of methylene acceptor;

[0187] the SBR is an SBR prepared in solution and the polybutadiene hasmore than 90% cis-1,4 bonds;

[0188] the reinforcing inorganic filler represents more than 90% of thetotal reinforcing filler.

[0189] The rubber compositions previously described, based on dieneelastomer, a high amount of reinforcing inorganic filler, a couplingagent and a methylene acceptor/donor system, in the proportionsindicated above, may advantageously constitute the entire treadaccording to the invention.

[0190] However, the invention also applies to those cases in which theserubber compositions comprising the M.A.D. system form only part of acomposite tread such as described for example in the introduction to thepresent specification, formed of two radially superposed layers ofdifferent rigidity (so-called “cap-base” structure), both intended tocome into contact with the road during running of the tire, during thelife of the latter.

[0191] The part based on the M.A.D. system may then constitute theradially outer layer of the tread which is intended to come into contactwith the ground from the start of running of the new tire, or on thecontrary its radially inner layer which is intended to come into contactwith the ground later, in those cases in which it is desired, forexample, to “retard” the technical effect of auto-accommodation providedby the invention.

[0192] Of course, the invention relates to the treads previouslydescribed, both in the uncured state (i.e. before curing) and in thecured state (i.e. after cross-linking or vulcanization).

III. EXAMPLES OF EMBODIMENT OF THE INVENTION

[0193] III-1. Preparation of the rubber compositions and treads

[0194] For the following tests, the procedure is as follows: thereinforcing filler, the coupling agent and any associated couplingactivator, the diene elastomer or the mixture of diene elastomers, themethylene acceptor and the various other ingredients, with the exceptionof the vulcanization system and the methylene donor, are introduced insuccession into an internal mixer filled to 70% of capacity, the initialtank temperature of which is approximately 60° C. Thermomechanicalworking (non-productive phase) is then performed in one stage, of aduration of about 3 to 4 minutes in total, until a maximum “dropping”temperature of 165° C. is obtained. In these tests, the methyleneacceptor is introduced into the mixer when the composition underkneading has reached a temperature close to 100° C.

[0195] The mixture thus obtained is recovered, it is cooled and then thesulphur, sulphenamide accelerator and methylene donor are incorporatedon an external mixer (homo-finisher) at 30° C., by mixing everything(productive phase) for an appropriate time, of between 5 and 12 minutesdepending on the case.

[0196] The compositions thus obtained are then calendered either in theform of plates (thickness of 2 to 3 mm) or of thin sheets of rubber inorder to measure their physical or mechanical properties, or extruded inthe form of treads for passenger-car tires.

[0197] In all the tests which follow, the reinforcing inorganic filler(silica type “HD”) is present in an amount greater than 60 phr; itfurthermore constitutes more than 90% by weight of all the reinforcingfiller, a minority fraction (less than 10%) of the latter beingconstituted by carbon black.

[0198] III-2. Tests

[0199] A) Test 1

[0200] In this first test, four rubber compositions are compared, basedon known SBR and BR diene elastomers, reinforced by carbon black orsilica, used as treads for tires for passenger vehicles.

[0201] These four compositions C-1 to C-4 are distinguished essentiallyby the following characteristics:

[0202] C-1: reinforced with carbon black (70 phr), without M.A.D.system;

[0203] C-2: reinforced with carbon black (70 phr), with M.A.D. system;

[0204] C-3: reinforced with silica (80 phr), without M.A.D. system;

[0205] C-4: reinforced with silica (80 phr), with M.A.D. system.

[0206] The compositions C-1 and C-3 constitute the controls “black” and“silica” of this test. Their respective formulations were adjusted to asto bring them both to initial iso-rigidity (Shore A hardness and modulusM10 equivalent), before incorporation of the M.A.D. system. CompositionC-3 furthermore contains the TESPT coupling agent (amount of 8% byweight relative to the quantity of silica) and the DPG (about 2.6% byweight relative to the quantity of silica). In this composition C-3, thecarbon black is essentially used as black pigmentation agent, and ispresent in a very small amount (6 phr, or approximately 7% of the totalreinforcing filler).

[0207] Compositions C-2 and C-4 correspond respectively to compositionsC-1 and C-3 to which the M.A.D. system has been added; only the treadcomprising composition C-4 is therefore in accordance with theinvention.

[0208] Tables 1 and 2 show the formulation of the different compositions(Table 1—amounts of the different products expressed in phr), and theirproperties before and after curing (40 min at 150° C.).

[0209] Comparison, first of all, of the control compositions C-1 and C-3(devoid of M.A.D. system) results in the following observations:

[0210] in the uncured state, a lower Mooney viscosity of composition C-2based on silica and a slightly increased scorching time, which isbeneficial to the processing of the composition;

[0211] after curing, an equivalent rigidity (identical Shore Ahardnesses; moduli at low deformation M10 very close) and identicalreinforcement properties (identical moduli at the high deformations(M100 and M300), equivalent mechanical properties at break, except forthe accuracy of measurement);

[0212] finally, expectedly, a lower hysteresis (lower values of ΔG* andtan(δ)_(max)) for composition C-2 based on silica, which is synonymouswith a lower rolling resistance.

[0213] After incorporation of the M.A.D. system, the followingmodifications of properties are observed for the two types ofcompositions (compare C-2 with C-1 on one hand, and C-4 with C-3 on theother hand):

[0214] in the uncured state, a Mooney viscosity which is littlemodified, a reduction in T5 in both cases (C-2 and C-4), with values T5(at least 10 min) which nevertheless offer a sufficient safety marginwith respect to the problem of scorching;

[0215] a substantial increase in the hysteresis in both cases; it willhowever be noted that the values of ΔG* and tan(δ)_(max) measured on thecomposition based on silica (C-4) remain at a low level, substantiallyequal to that of the control composition C-1 filled with carbon black,which constitutes a significant advantage for the silica-based tread;

[0216] finally, a great rise in the value of modulus at low deformation(values M10 practically doubled) and in the Shore hardness (increased byabout 20%).

[0217] These results may be regarded as expected. In particular, thevery distinct increase in rigidity (Shore hardness and modulus at lowdeformation M10), due to the presence of the M.A.D. system in thesecompositions C-2 and C-4, enables the person skilled in the art toexpect, for tires mounted on automobiles the treads of which are formedof such compositions, certainly an improvement in the road behaviourowing to an increased drift thrust, but above all a crippling drop inthe grip performance on wet ground.

[0218] However, one significant difference must be noted betweencompositions C-2 and C-4, this difference relating to the evolution ofthe modulus (M¹⁰ _(Ac)) at low deformation, after mechanicalaccommodation (15%).

[0219] In the case of the control composition C-2 (carbon black), itwill be noted that the modulus M10_(Ac) remains very high afteraccommodation (9.1 MPa compared with an initial 5.5 MPa on compositionC-1, or approximately 65% greater); on the contrary, this same modulusM10_(AC) drops very greatly (from 12.0 MPa to 7.2 MPa) for compositionC-4 (silica), recovering practically the initial value M10 (6.0 MPa)recorded for the control composition C-3 devoid of M.A.D. system.

[0220] Such a difference in response between the two compositions, inthe presence of the M.A.D. system, is totally unexpected; it justifiesthese first results now facing real tire running tests, as set forth inTest 2 below.

[0221] B) Test 2 (running tests for tires)

[0222] Compositions C-1 to C-4 previously described are used in thistest as treads for radial carcass passenger-car tires, of dimension195/65 R15 (speed index H), conventionally manufactured and identical inall points except for the rubber composition constituting the tread.

[0223] These tires are marked P-1 to P-4 and correspond to compositionsC-1 to C-4, respectively; they were first tested on a machine todetermine their initial Shore A hardness (on tread) and their driftthrust.

[0224] They were then mounted on a passenger vehicle in order to besubjected to the braking and grip tests described in section 1-6 above,in accordance with the following specific conditions:

[0225] braking on damp ground: vehicle Renault model Laguna (front andrear pressure: 2.0 bar), the tires to be tested being mounted at thefront of the vehicle;

[0226] travel on a damp circuit comprising bends: vehicle BMW model 328(front pressure 2.0 bar; rear pressure: 2.4 bar), the tires to be testedbeing mounted at the front and at the rear of the vehicle.

[0227] Finally, their Shore A hardness was also measured after “standardrunning-in” (see (C) of section 1-6) on the Renault Laguna vehicle.

[0228] The results obtained, set forth in Table 3, result in thefollowing comments:

[0229] it will be noted first of all that the Shore A hardnessesmeasured right at the surface of the treads of the new tires (“initial”Shore A hardnesses) are virtually equal to those measured on rubber testpieces (corresponding compositions C-1 to C-4—see Table 2 above),whatever the type of reinforcing filler used (carbon black or silica);the addition of the M.A.D. system results in both cases in a distinctincrease in Shore hardness, of about 13 points (from 67 points to 79-80points);

[0230] this increase in rigidity of the tread, for the two types oftires P-2 and P-4, is accompanied predictably by a very great increasein the drift thrust (+30%), which is a clear indicator to the personskilled in the art of improved road behaviour (on dry ground);

[0231] as far as the braking performance on damp ground is concerned, itwill be noted first of all that the control tires P-1 and P-3, which aredevoid of the M.A.D. system, exhibit an equivalent performance (base 100used for the tire P-1);

[0232] after incorporation of the M.A.D. system, the control tire P-2,the tread of which is reinforced with carbon black, exhibits a cripplingdrop (20% loss) in this braking performance, which was furthermoreentirely predictable, taking into account the great stiffening of thetread provided by the M.A.D. system;

[0233] the unexpected result lies in the behaviour of the tire of theinvention P-4, the tread of which is reinforced with silica: not only isits braking performance on wet ground not degraded—which is already avery surprising result for the person skilled in the art—but it issubstantially improved since an improvement of 8% is obtained relativeto the control tires P-1 and P-3; this behaviour is entirely noteworthyand radically opposed to that of the control tire P-2;

[0234] as for the running test on a damp circuit comprising bends, itconfirms that the incorporation of the M.A.D. system in the conventionaltread filled with carbon black (tire P-2) results in an unacceptabledrop in grip, which is illustrated both by an increase in the minimumtime necessary to travel round the circuit at limit speed conditions(plus 7%) and by a reduction in the behaviour mark attributed by thedriver (drop of 25%, which is very significant for such a test);

[0235] whereas the incorporation of the same M.A.D. system in the treadof the invention filled with silica (tire P-4) results on the contraryin a very significant improvement in performance: time to travel roundthe circuit significantly reduced (improvement of 3%) compared with thecontrol tires, better behaviour mark (improvement of 10%);

[0236] correlatively to the running results above, it will be noted thatafter mechanical accommodation (“standard running-in”), the Shore Asurface hardness remains unchanged (except for the accuracy ofmeasurement) on the control tires P-1, P-2 and P-3, whereas it dropsvery significantly (less 10 points) on the tire P-4 according to theinvention; this remarkable behaviour furthermore recalls the evolutionof the modulus M10_(Ac) observed on the corresponding rubbercompositions (C-1 to C-4) in Test 1 above.

[0237] In summary, the tire according to the invention P-4, entirelyunexpectedly, exhibits a simultaneous increase in two properties whichhowever have been deemed contradictory, namely road behaviour (driftthrust) and grip on wet ground.

[0238] It must be deduced from this that, necessarily, thethree-dimensional resin lattice provided by the M.A.D. system in therubber compositions of the treads is “expressed” differently dependingon whether these compositions are filled conventionally with carbonblack, or rather with a reinforcing inorganic filler such as silica, inthe recommended high amount.

[0239] C) Test 3

[0240] This third test, a posteriori, provides an explanation for theimproved results of Tests 1 and 2 above, by revealing an unexpectedproperty for the tread of the invention: the latter in fact has, in theradial direction, a very marked rigidity gradient, with an increasingrigidity when moving from the surface towards the inside of the tread;such a characteristic does not exist in the control tread filled withcarbon black.

[0241] The single attached FIGURE represents, for the treads of tiresP-2 and P-4 of the previous test, the evolution of modulus M10 (in MPa)as a function of the depth “e” (from 0 to 6 mm), before (as-new state)and after “standard running-in” of these tires on the aforementionedRenault Laguna vehicle.

[0242] In order to obtain these rigidity profiles, strips of tread werecut out, at regular intervals of depth “e” (for example every 2/10 mm),practically over the entire thickness of this tread (approximately 7mm); then these strips were subjected to traction to determine theirmodulus M10 as a function of their depth “e” in the tread, measuredrelative to the surface (e=0 mm) of the latter.

[0243] More precisely, there are the following correspondences:

[0244] the curve A corresponds to the control tires P-1 and P-3, that isto say to the treads without M.A.D. system, the profiles of modulusessentially coinciding (except for the accuracy of measurement) betweenthe two types of reinforcing filler used (carbon black or silica);

[0245] the curve B corresponds to the tires P-2 and P-4, that is to sayto the treads with M.A.D. system, these tires being new, that is to saybefore any mechanical running-in or accommodation; here too, theprofiles of modulus essentially coincide between carbon black andsilica;

[0246] the curve C corresponds to the tire P-2 (carbon black) afterrunning-in;

[0247] the curve D corresponds to the tire P-4 (silica) afterrunning-in.

[0248] After running-in, radically different behaviour is observedbetween the tires P-2 and P-4:

[0249] the surface rigidity and the rigidity at depth are littledifferent for the tire P-2 of the prior art, the reinforcing filler ofwhich is carbon black;

[0250] whereas, in the case of the tire P-4 of the invention, thesurface rigidity is very significantly less, practically equal to thatof the control tires P-1 and P-3, with furthermore a very marked radialrigidity gradient, which increases on moving from the surface of thetread towards the inside of the latter.

[0251] It should perhaps be deduced from the curves A to D above, andfrom all the above results, that the three-dimensional stiffeninglattice formed by the M.A.D. system has lower solidity in the case ofthe tread filled with silica than in the case of the conventional treadfilled with carbon black.

[0252] Due to this relative fragility, stresses of low amplitude,typical of those experienced during running by the surface part of thetread, would be sufficient to break the surface resin lattice, and thusto make the surface part of the tread more flexible and less rigid, andthus make it recover the excellent grip performance which it has in theabsence of the M.A.D. system. On the other hand, in depth, the latticeresin would be little affected by running, all the more so as onepenetrates inside this tread, thus guaranteeing the additional rigiditysufficient for improved road behaviour (greater drift thrust).

[0253] D) Test 4

[0254] In this new test, five rubber compositions are compared, allreinforced with silica (80 phr) and a small proportion (6 phr) of carbonblack as black pigmentation agent, these compositions also being used astreads for tires for passenger vehicles.

[0255] The control composition (C-5) is devoid of M.A.D. system, whereasthe other four (C-6 to C-9) comprise such a system; the methyleneacceptor is formed by different variants of novolac resins (6 phr), themethylene donor being HMT (2 phr). Each composition comprises inparticular a coupling agent for the silica. The treads comprising thecompositions C-6 to C-9 are therefore all in accordance with theinvention. Tables 4 and 5 show the precise formulation of the differentcompositions (Table 4—amounts of the different products expressed inphr), and their properties before and after curing (40 min at 150° C).

[0256] On reading these results, it will be noted that, compared withthe control composition C-5 which is devoid of M.A.D. system, thecompositions used as treads according to the invention have thefollowing characteristics:

[0257] in the uncured state, a lower Mooney viscosity, which isbeneficial to processing; a reduction in T5 in all cases, with values(10 to 13 min) which nevertheless offer a sufficient safety margin withrespect to the problem of scorching;

[0258] after curing, reinforcement properties which are substantiallyequivalent, illustrated by values close to the moduli at highdeformations (M100 and M300), and also the properties at break;

[0259] significantly greater rigidity, illustrated both by a Shore Ahardness increased by approximately 16% (change from 67 points to anaverage value of 78 points) and by a modulus M10 at low deformationwhich is substantially doubled (from 6.3 MPa to 12.5 MPa, on average);

[0260] correlatively, an expected increase in the hysteresis (increasein dynamic properties ΔG* and tan(δ)_(max));

[0261] finally, after mechanical accommodation, values of modulus at lowdeformation (M10_(Ac)) which drop very greatly, recovering values closedto the initial value observed on the control composition C-5 which isdevoid of resin lattice.

[0262] The unexpected results of Test 1 above (virtual “reversibility”of the modulus M10, after mechanical accommodation) are thus clearlyconfirmed, in the presence of various types of novolac resins, making itpossible to predict, for tires comprising these compositions C-6 to C-9as treads, the same improved compromise of road behaviour and grip asthat observed in Test 2 above, owing to the presence of a radialrigidity gradient in the tread.

[0263] It will furthermore be noted that the compositions C-6 to C-9according to the invention have very close properties, whatever thenovolac resin used as methylene acceptor.

[0264] E) Test 5

[0265] Here five compositions filled with silica (C-10 to C-14) arecompared, similar to those of Test 4 above.

[0266] A first control composition (C-10) is devoid of M.A.D. system, asecond control composition (C-11) comprises the methylene acceptor butnot the methylene donor. The other three compositions (C-12 to C-14)constitute three new variants, with different M.A.D. systems, ofcompositions usable as treads according to the invention; it will benoted in particular that composition C-14 comprises, as M.A.D. system,two different methylene acceptors and two different methylene donors.

[0267] Tables 6 and 7 show the formulation of the different compositions(Table 6—amounts of the different products expressed in phr), and theirproperties before and after curing (40 min at 150° C.).

[0268] Reading these results entirely confirms, if it were necessary,the conclusions of the above tests, namely, for the compositions C-12,C-13 and C-14 according to the invention, compared with the controlC-10;

[0269] in the uncured state, a lower Mooney viscosity, which isbeneficial to processing;

[0270] admittedly a reduction in T5, but values (15 min) which aresatisfactory with respect to the problem of scorching;

[0271] after curing, an (expected) increase in the hysteresis (ΔG* andtan(δ)_(max));

[0272] reinforcement properties which are at least equal (values closeto the moduli M100 and M300; equivalent properties at break);

[0273] distinctly increased rigidity: plus 10 points on the Shorehardness, modulus M10 virtually doubled (from 6.3 MPa to 10.8 MPa onaverage);

[0274] finally, after accommodation, values of modulus at lowdeformation (M10_(Ac)) which practically recover those of the controlC-10.

[0275] In accordance with the results of the preceding tests, theselatter two characteristics must be analysed as synonymous of improvedroad behaviour (due to the stiffening in depth of the tread) withoutadversely affecting or even improving the grip on wet ground (due to asurface rigidity which is virtually not modified, after accommodation).

[0276] It will be noted finally that the control composition C-11,comprising a methylene acceptor without curing agent, revealsintermediate properties which are of little interest.

[0277] As for the composition C-14 according to the invention,comprising two different methylene acceptors(formo-phenol+diphenylolpropane) associated with two methylene donors(HMT and H3M), it exhibits the best overall compromise of properties forthis test, both before curing (viscosity and scorching time) and aftercuring (rigidity, reinforcement and hysteresis).

[0278] F) Test 6

[0279] In this test two new rubber compositions based on silica (C-15and C-16) are compared which constitute two new variants of compositionsusable as treads according to the invention. The amount of reinforcinginorganic filler has been slightly reduced compared with the precedingtests, while remaining within a preferred range of between 60 phr and100 phr.

[0280] The composition C-16 is identical to composition C-15, exceptthat a coupling activator system such as described in the aforementionedapplication WO00/05301, formed by the association of a zincdithiophosphate (DTPZn) and a guanidine derivative (DPG) has been addedto C-16; such an activator has the ability to improve the effectivenessof a polysulphurized alkoxysilane coupling agent.

[0281] Tables 8 and 9 show the formulation of the differentcompositions, and their properties before and after curing (40 min at150° C.).

[0282] On reading these results, it will be noted that the incorporationof DTPZn and DPG in composition C-16 has a beneficial effect on themajor part of the properties, with in particular:

[0283] an increase in the reinforcement, as indicated by a substantialincrease in the values M100 and M300, and in the ratio M300/M100;

[0284] advantageously combined with a substantial reduction in thehysteresis (reduction of ΔG* and tan(δ)_(max)).

[0285] The two evolutions above clearly illustrate a better interactionbetween the reinforcing inorganic filler and the elastomer, in otherwords a coupling effect which is optimized due to the presence of thecoupling activator system.

[0286] It will furthermore be noted that this optimized reinforcement isobtained without adversely affecting the action of the M.A.D. system(values of Shore A hardness, moduli M10 and M10_(Ac) identical).

[0287] In the preceding results a slight increase in the hysteresis inthe presence of the M.A.D. system had been noted, for the compositionsand treads of the invention filled with silica, which is harmful withrespect to the rolling resistance. The use of a coupling activator is away of overcoming this drawback owing to the possibility which itoffers, for example, of reducing the amount of reinforcing inorganicfiller.

[0288] G) Test 7

[0289] Five rubber compositions are compared, all reinforced with silica(80 phr) and a small proportion (6 phr) of carbon black, thesecompositions being used as treads for tires for passenger vehicles.

[0290] The control composition (C-17) is devoid of M.A.D. system. Theother 4 compositions (C-18 to C-21) comprise such a system; themethylene acceptor is used therein in amounts varying from 2 to 5 phr,the quantity of methylene donor being selected to be equal to 60% byweight relative to the quantity of acceptor.

[0291] Tables 10 and 11 give the formulation of the differentcompositions, some of their properties after curing (40 min at 150° C.),and the properties of the tires (respectively P-17 to P-21) comprisingthe corresponding treads. These tires are manufactured and tested asindicated previously for Test 2.

[0292] The treads comprising the compositions C-18 to C-21 and the tiresP-18 to P-21 are therefore all in accordance with the invention.

[0293] On reading the results of Table 11, it will be noted first of allthat, compared with the control composition C-17, the compositions C-18to C-21 have, all the more so the larger the quantity of M.A.D. system,a greater rigidity illustrated by a Shore A hardness and a modulus M10both of which are significantly increased, the modulus M10 beingsubstantially doubled (from 5.6 MPa to 11.3 MPa) for the highest amountof M.A.D. system.

[0294] This greater rigidity results, for the tires according to theinvention, not only in an (expected) increase in the drift thrust (anindicator of the behaviour on dry ground), but also and in particular,unexpectedly, in an improvement in the behaviour on wet ground (seeperformance on a damp circuit comprising bends).

[0295] There are thus confirmed, in the presence of the M.A.D. systemcombined with the high amount of reinforcing inorganic filler, theimproved overall compromise of road behaviour and grip as observed inTest 2 above, obtained due to the presence of a radial rigidity gradientin the treads according to the invention.

[0296] H) Test 8

[0297] In this Test 2 new compositions are compared which are reinforcedwith a high amount of silica, used to constitute all or of a tread forpassenger-vehicle tires.

[0298] The composition according to the invention (C-23) comprises theM.A.D. system, whereas the control composition (C-22) is devoid thereof.The formulation of the 2 compositions is close to those of Table 6 (Test5), except that, in particular, the diene elastomer used here is amixture of two SBRs of different microstructures.

[0299] Tables 12 and 13 give this detailed formulation of the 2compositions, some of their properties after curing (40 min at 150° C.),and the properties of the corresponding tires (respectively P-22 andP-23), manufactured and tested as indicated for Test 2 above, except forthe following details:

[0300] the tread of the control tires P-22 is formed exclusively by thecontrol composition C-22;

[0301] the tread of the tires P-23 according to the invention is acomposite tread having a “cap/base” structure such as described above,formed by the two radially superposed compositions C-22 and C-23, thecomposition according to the invention C-23 constituting the base, thatis to say the radially inner part (on a new tire) of the tread.

[0302] On reading the results of Table 13, it will be noted first of allthat the presence of the M.A.D. system in the composition C-23 involvesa very substantial increase in Shore A hardness (nearly 20% greater).This greater rigidity results, for the tires P-23 according to theinvention, in significantly improved on-vehicle behaviour, not only ondry ground (increase in the drift thrust) but also and in particular onwet ground: travelling time distinctly reduced (less than 4 secondsapproximately per lap of the circuit) and a significantly improvedbehaviour mark (+15%).

[0303] Thus, the beneficial effect of the invention, obtained due to thephenomenon of auto-accommodation, is once again demonstrated, even inthe case in which the composition based on the M.A.D. system constitutesonly part of the tread, in this case in this test the radially innerpart of the tread which is intended to come into contact with the roadonly later, after its (radially) outer part of this tread has becomeworn.

[0304] I) Test 9

[0305] In this last test, 2 new compositions reinforced with silica arecompared, which are similar to those of Test 1 (compositions C-3 andC-4) except that, in particular, in this case an HD silica of smallerspecific surface area is used. The composition according to theinvention (C-25) comprises the M.A.D. system, whereas the controlcomposition (C-24) is devoid thereof, these 2 compositions are used astreads for passenger-car tires.

[0306] Tables 14 and 15 give the formulation of the compositions, someof their properties before and after curing (40 min at 150° C.), and theproperties of the corresponding tires (respectively P-24 and P-25)(manufactured and tested as indicated for Test 2 above).

[0307] The results of Table 15 confirm once again the beneficialunexpected effects of the invention (composition C-25 and tires P-25),due to the combined presence of the M.A.D. system and the high amount ofreinforcing inorganic filler, with:

[0308] a significant reduction in viscosity in the uncured state;

[0309] a very substantial increase in the rigidity in the cured state(Shore A hardness and modulus M10), without adversely affecting thelevel of reinforcement (illustrated by the modulus M100);

[0310] finally, once again, significantly improved behaviour of thetires according to the invention, both on dry ground (drift thrustincreased by 15%) and on wet ground, due to the auto-accommodationresulting from the combined presence of the M.A.D. system and a highamount of reinforcing inorganic filler.

[0311] In conclusion, due to the treads according to the invention andto the specific formulation of their rubber compositions, it ishenceforth possible to “reconcile” what had hitherto appearedirreconcilable, namely grip on one hand and road behaviour on the otherhand, without using solutions which are complex, costly or non-durablesuch as described in the introduction to the present specification.

[0312] The treads according to the invention described in the precedingexamples offer the major advantage, compared with the “composite” treadsof the prior art, on one hand of maintaining their compromise ofperformances throughout the life of the tire, due to the unexpectedphenomenon of auto-accommodation, and on the other hand of having a trueradial rigidity gradient, and not a simple, very localized, “accident”of rigidity. This true rigidity gradient results in optimum “working” ofthe blocks of rubber in contact with the ground, during running and thenumerous forces transmitted to the tread, in other words is synonymouswith a tire which grips the road even better.

[0313] The invention will find a very advantageous application in tiresfitted on vehicles such as motorcycles, passenger cars, vans or heavyvehicles, in particular in high-grip tires of the “snow” or “ice” type(also referred to as “winter” tires) which, owing to a deliberately moreflexible tread, could hitherto have qualities of road behaviour, on dryground, which were sometimes judged to be insufficient.

[0314] The novel compromise of properties thus obtained, which hasclearly shifted compared with the acquired knowledge of the prior art,may furthermore be obtained while keeping the performances of rollingresistance and of wear resistance at the high levels which one isentitled to expect nowadays of rubber compositions based on reinforcinginorganic fillers such as highly dispersible silicas, capable ofreplacing the conventional tire-grade carbon blacks. TABLE 1 CompositionNo.: C-1 C-2 C-3 C-4 SBR (1) 70 70 70 70 BR (2) 30 30 30 30 carbon black(3) 70 70 6 6 silica (4) — — 80 80 coupling agent (5) — — 6.4 6.4methylene acceptor (6) — 6 — 6 oil (7) 20 20 39 39 DPG (8) — — 2.1 2.1ZnO 2.5 2.5 2.5 2.5 stearic acid 2 2 2 2 antioxidant (9) 1.9 1.9 1.9 1.9methylene donor (10) — 2 — 2 sulphur 1.5 1.5 1.5 1.5 accelerator (11)1.5 1.5 1.5 1.5

[0315] TABLE 2 Composition No.: C-1 C-2 C-3 C-4 Properties beforecuring: Mooney (MU) 100 105 75 70 T5 (min) 15 10 19 13 Properties aftercuring: M10 (MPa) 5.5 11.8 6.0 12.0 M100 (MPa) 1.85 2.3 1.9 2.2 M300(MPa) 2.2 2.0 2.15 1.6 Shore A 65 79 66 80 ΔG* 10 17 6.8 10 tan(δ)_(max)0.38 0.43 0.32 0.39 M10_(Ac) 4.4 9.1 4.8 7.2 Breaking stress (MPa) 20 1919 15 Elongation at break (%) 590 550 540 620

[0316] TABLE 3 Tires: P-1 P-2 P-3 P-4 Initial Shore A hardness:  67  79 67  80 (evolution in r.u.) (100) (118) (100) (119) Drift thrust( r.u.):100 130 100 130 Braking on damp ground 100  80 100 108 (r.u.):Performance on a damp circuit comprising bends (r.u.): travelling time:100 107 100  97 behaviour mark: 100  75 100 110 Shore A hardness after 67  78  66  70 “standard running-in”: (100) (116) (99) (106) (evolutionin r.u.)

[0317] TABLE 4 Composition No.: C-5 C-6 C-7 C-8 C-9 SBR (1) 60 60 60 6060 BR (2) 40 40 40 40 40 carbon black (3) 6 6 6 6 6 silica (4) 80 80 8080 80 coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor (6) — 66 6 6 oil (7) 39 39 39 39 39 DPG (8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.5 2.5 2.52.5 2.5 stearic acid 2 2 2 2 2 antioxidant (9) 1.9 1.9 1.9 1.9 1.9methylene donor (10) — 2 2 2 2 sulphur 1.1 1.1 1.1 1.1 1.1 accelerator(11) 2 2 2 2 2

[0318] TABLE 5 Composition No.: C-5 C-6 C-7 C-8 C-9 Properties beforecuring: Mooney (MU) 93 81 74 76 79 T5 (min) 18 12 10 13 11 Propertiesafter curing: M10 (MPa) 6.3 12.4 13.7 12.0 11.6 M100 (MPa) 1.8 2.1 2.12.1 2.1 M300 (MPa) 2.0 1.8 1.8 1.85 1.85 Shore A 67 78 79.5 77 77 ΔG*6.1 12.4 11.8 12.7 12.0 tan(δ)_(max) 0.32 0.37 0.38 0.39 0.38 M10_(Ac)4.9 7.5 8.2 7.1 7.1 Breaking stress (MPa) 21 18 19 18 17 Elongation atbreak (%) 595 610 630 575 560

[0319] TABLE 6 Composition No.: C-10 C-11 C-12 C-13 C-14 SBR (1) 70 7070 70 70 BR (2) 30 30 30 30 30 carbon black (3) 6 6 6 6 6 silica (4) 8080 80 80 80 coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor(6) — 5 5 5 1.5 methylene acceptor (6 bis) — — — — 2.5 oil (7) 33 33 3333 33 DPG (8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.5 2.5 2.5 2.5 2.5 stearic acid 22 2 2 2 antioxidant (9) 1.4 1.4 1.4 1.4 1.4 methylene donor (10) — — —1.5 0.75 methylene donor (12) — — 1.5 — 1.5 sulphur 1.5 1.5 1.5 1.5 1.5accelerator (11) 1.5 1.5 1.5 1.5 1.5

[0320] TABLE 7 Composition No.: C-10 C-11 C-12 C-13 C-14 Propertiesbefore curing: Mooney (MU) 90 91 84 85 78 T5 (min) 17 11 14 14 14Properties after curing: M10 (MPa) 6.3 9 12 11.2 11 M100 (MPa) 1.9 2.02.5 2 2.2 M300 (MPa) 2.1 2.0 2.3 1.7 2.1 Shore A 68 72 78.5 78 77 ΔG*6.8 11 15 12.4 12.9 tan(δ)_(max) 0.34 0.41 0.40 0.37 0.40 M10_(Ac) 5.06.6 7.3 6.9 6.7 Breaking stress (MPa) 19 20 18 17 18 Elongation at break(%) 540 585 510 600 550

[0321] TABLE 8 Composition No.: C-15 C-16 SBR (1) 70 70 BR (2) 30 30carbon black (3) 6 6 silica (4) 70 70 coupling agent (5) 5.6 5.6methylene acceptor (6) 4 4 oil (7) 25 25 DTPZn (10) — 0.75 DPG (8) 1.31.3 ZnO 2.5 2.5 stearic acid 2 2 antioxidant (9) 1.9 1.9 methylene donor(12) 1 1 sulphur 1.5 1.5 accelerator (11) 1.5 1.5

[0322] TABLE 9 Composition No.: C-15 C-16 Properties before curing:Mooney (MU) 88 88 T5 (min) 16 10 Properties after curing: M10 (MPa) 9.29 M100 (MPa) 2.1 2.5 M300 (MPa) 2.1 2.9 Shore A 74 74.5 ΔG* 10 8tan(δ)_(max) 0.35 0.32 M10_(Ac) 5.7 5.6 Breaking stress (MPa) 19 17Elongation at break (%) 585 485

[0323] TABLE 10 Composition No.: C-17 C-18 C-19 C-20 C-21 SBR (1) 70 7070 70 70 BR (2) 30 30 30 30 30 carbon black (3) 6 6 6 6 6 silica (4) 8080 80 80 80 coupling agent (5) 6.4 6.4 6.4 6.4 6.4 methylene acceptor(6) — 5 4 3 2 oil (7) 33 33 33 33 33 DPG (8) 1.5 1.5 1.5 1.5 1.5 ZnO 2.52.5 2.5 2.5 2.5 stearic acid 2 2 2 2 2 antioxidant (9) 1.4 1.4 1.4 1.41.4 methylene donor (10) — 3 2.4 1.8 1.2 sulphur 1.1 1.1 1.1 1.1 1.1accelerator (11) 2.0 2.0 2.0 2.0 2.0

[0324] TABLE 11 Composition No. C-17 C-18 C-19 C-20 C-21 M10 (MPa) 5.611.3 9.9 8.6 7.8 Shore A 70 82 79 77 74 Tire No.: P-17 P-18 P-19 P-20P-21 Drift thrust (r.u.) 100 122 116 111 109 Performance on a dampcircuit comprising bends (r.u.): travelling time: 100 98.5 99.2 98.499.5 behaviour mark: 100 115 115 115 105

[0325] TABLE 12 Composition No. C-22 C-23 SBR (1) 55 55 SBR (2) 45 45carbon black (3) 6 6 silica (4) 82 82 coupling agent (5) 6.6 6.6methylene acceptor (6) — 1.5 methylene acceptor (6 bis) — 2.5 oil (7)41.5 41.5 DPG (8) 1.5 1.5 ZnO 2.5 2.5 stearic acid 2 2 antioxidant (9)1.9 1.9 methylene donor (10) — 0.75 methylene donor (12) — 1.5 sulphur1.4 1.4 accelerator (11) 1.5 1.5

[0326] TABLE 13 Composition No. C-22 C-23 Shore A  72  82 (evolution inr.u.) (100) (119) Tire No.: P-22 P-23 Drift thrust (r.u.): 100 125Performance on a damp circuit comprising bends (r.u.): travelling time:100  96 behaviour mark: 100 115

[0327] TABLE 14 Composition No. C-24 C-25 SBR (1) 70 70 BR (2) 30 30carbon black (3) 6 6 silica (4) 80 80 coupling agent (5) 4.8 4.8methylene acceptor (6) — 5 oil (7) 33 33 DPG (8) 1.1 1.1 ZnO 2.5 2.5stearic acid 2 2 antioxidant (9) 1.9 1.9 methylene donor (10) — 3sulphur 1.1 1.1 accelerator (11) 2 2

[0328] TABLE 15 Composition No. C-24 C-25 Mooney (MU) 71 64 M10 (MPa)5.5 10.9 M100 (MPa) 1.8 1.9 Tire No.: P-22 P-23 Drift thrust (r.u.): 100115 Performance on a damp circuit comprising bends (r.u.): travellingtime: 100  97 behaviour mark: 100 110

1. A tire tread comprising a rubber composition based on: (i) a dieneelastomer; (ii) more than 50 phr of an inorganic filler as reinforcingfiller; (iii) between 2 and 15 phr of an (inorganic filler/dieneelastomer) coupling agent; (iv) between 1 and 10 phr of a methyleneacceptor, and (v) between 0.5 and 5 phr of a methylene donor.
 2. Thetread according to claim 1, wherein the diene elastomer is selected fromthe group which consists of polybutadienes, synthetic polyisoprenes,natural rubber, butadiene copolymers, isoprene copolymers and mixturesof these elastomers.
 3. The tread according to claim 2, wherein thecopolymers are selected from the group which consists ofbutadiene-styrene copolymers, butadiene-isoprene copolymers,isoprene-styrene copolymers, butadiene-styrene-isoprene copolymers, andmixtures of these copolymers.
 4. The tread according to claim 1 or 2,wherein the reinforcing inorganic filler is a siliceous or aluminousfiller.
 5. The tread according to claim 4, wherein the reinforcinginorganic filler is silica.
 6. The tread according to claim 1, whereinthe quantity of reinforcing inorganic filler is between 60 and 100 phr.7. The tread according to claim 1, wherein the inorganic filler-dieneelastomer coupling agent is an organosilane or a polyorganosiloxane. 8.The tread according to claim 7, wherein the coupling agent is apolysulphurized alkoxysilane.
 9. The tread according to claim 1, whereinthe quantity of methylene acceptor is between 2 and 8 phr.
 10. The treadaccording to claim 1, wherein the quantity of methylene donor is between0.5 and 3.5 phr.
 11. The tread according to claim 1, wherein themethylene acceptor is selected from the group consisting of phenolicresins.
 12. The tread according to claim 11, wherein the methyleneacceptor is a novolac phenolic resin.
 13. The tread according to claim1, wherein the methylene donor is selected from the group consisting ofhexamethylenetetramine (HMT), hexamethoxymethylmelamine (H3M),hexaethoxymethylmelamine, polymers of para-formaldehyde, N-methylolderivatives of melamine, and mixtures of these compounds.
 14. The treadaccording to claim 13, wherein the methylene donor is selected from thegroup consisting of HMT, H3M, and mixtures of these compounds.
 15. Thetread according to claim 1, wherein the quantity of methylene acceptorrepresents between 2% and 15% by weight relative to the weight ofreinforcing inorganic filler.
 16. The tread according to claim 1,wherein the quantity of methylene donor represents between 10% and 80%by weight relative to the weight of methylene acceptor.
 17. The treadaccording to claim 2, wherein the diene elastomer is a butadiene-styrenecopolymer (SBR).
 18. The tread according to claim 17, wherein the SBRelastomer has a styrene content of between 20% and 30% by weight, acontent of vinyl bonds of the butadiene fraction of between 15% and 65%,a content of trans-1,4 bonds of between 20% and 75% and a glasstransition temperature of between −20° C. and −55° C.
 19. The treadaccording to claim 17, wherein the SBR is an SBR prepared in solution.20. The tread according to claim 17, wherein the SBR is used in amixture with a polybutadiene.
 21. The tread according to claim 20,wherein the polybutadiene has more than 90% cis-1,4 bonds.
 22. The treadaccording to claim 1, wherein the reinforcing inorganic fillerrepresents more than 80% of the total reinforcing filler.
 23. The treadaccording to claim 1, wherein the reinforcing inorganic filler is usedin a mixture with carbon black.
 24. The tread according to claim 23,wherein the carbon black is present in an amount of between 2 and 15phr.
 25. The tread according to claim 24, wherein the carbon black ispresent in an amount of between 4 and 12 phr.
 26. The tread according toclaim 8, wherein the coupling agent is selected from the groupconsisting of bis-(C₁-C₄)alkoxylsilyl(C₁-C₁₀)alkyl polysulphides. 27.The tread according to claim 26, wherein the coupling agent is abis-(C₁-C₄)alkoxylsilylpropyl polysulphide.
 28. The tread according toclaim 27, wherein the coupling agent is bis 3-triethoxysilylpropyldisulphide or tetrasulphide.
 29. The tread according to claim 1, whereinsaid tread comprises a structure of the “cap/base” type formed of twodifferent, radially superposed, rubber compositions, and wherein therubber composition which comprises the methylene acceptor and themethylene donor forms the radially inner part of this tread.
 30. Thetread according to claim 1, wherein said tread comprises a structure ofthe “cap/base” type formed of two different, radially superposed, rubbercompositions, and wherein the rubber composition which comprises themethylene acceptor and the methylene donor forms the radially outer partof this tread.
 31. The tread according to claim 1, wherein said tread isin the vulcanized state.
 32. A process for preparing asulphur-vulcanizable tire tread having, after mechanical running-in ofthe tire for which it is intended, a rigidity gradient which increasesradially from the surface towards the inside of the tread, characterizedin that said process comprises the following steps: incorporating in adiene elastomer, in a mixer: more than 50 phr of an inorganic filler asreinforcing filler; between 2 and 15 phr of an inorganic filler-dieneelastomer coupling agent providing the connection between thereinforcing inorganic filler and the diene elastomer; between 1 and 10phr of a methylene acceptor, by thermomechanically kneading the entiremixture, in one or more stages, until a maximum temperature of between130° C. and 200° C. is reached; cooling the entire mixture to atemperature of less than 100° C.; then incorporating: between 0.5 and 5phr of a methylene donor, a vulcanization system; kneading the entiremixture until a maximum temperature of less than 120° C.; extruding orcalendering the rubber composition thus obtained, in the form of a tiretread.
 33. The process of claim 32, wherein the diene elastomer isselected from the group which consists of polybutadienes, syntheticpolyisoprenes, natural rubber, butadiene copolymers, isoprene copolymersand mixtures of these elastomers.
 34. The process according to claim 32,wherein the reinforcing inorganic filler is a siliceous or aluminousfiller.
 35. The process according to claim 34, wherein the reinforcinginorganic filler is silica.
 36. The process according to claim 32,wherein the quantity of reinforcing inorganic filler is between 60 and100 phr.
 37. The process according to claim 32, wherein the inorganicfiller-diene elastomer coupling agent is an organosilane or apolyorganosiloxane.
 38. The process according to claim 37, wherein thecoupling agent is a polysulphurized alkoxysilane.
 39. The processaccording to claim 32, wherein the quantity of methylene acceptor isbetween 2 and 8 phr.
 40. The process according to claim 32, wherein thequantity of methylene donor is between 0.5 and 3.5 phr.
 41. The processaccording to claim 32, wherein the methylene acceptor is selected fromthe group consisting of phenolic resins.
 42. The process according toclaim 41, wherein the methylene acceptor is a novolac phenolic resin.43. The process according to claim 32, wherein the methylene donor isselected from the group consisting of hexamethylenetetramine (HMT),hexamethoxymethylmelamine (H3M), hexaethoxymethylmelamine, polymers ofpara-formaldehyde, N-methylol derivatives of melamine, and mixtures ofthese compounds.
 44. The process according to claim 43, wherein themethylene donor is selected from among HMT, H3M, and mixtures of thesecompounds.
 45. The process according to claim 32, wherein the quantityof methylene acceptor represents between 2% and 15% by weight relativeto the weight of reinforcing inorganic filler.
 46. The process accordingto claim 32, wherein the quantity of methylene donor represents between10% and 80% by weight relative to the weight of methylene acceptor. 47.The process according to claim 33, wherein the diene elastomer is abutadiene-styrene copolymer (SBR).
 48. The process according to claim47, wherein the SBR elastomer has a styrene content of between 20% and30% by weight, a content of vinyl bonds of the butadiene fraction ofbetween 15% and 65%, a content of trans-1,4 bonds of between 20% and 75%and a glass transition temperature of between −20° C. and −55° C. 49.The process according to claim 48, wherein the SBR is an SBR prepared insolution.
 50. The process according to claim 49, wherein the SBR is usedin a mixture with a polybutadiene.
 51. The process according to claim50, wherein the polybutadiene has more than 90% cis-1,4 bonds.
 52. Theprocess according to claim 32, wherein the reinforcing inorganic fillerrepresents more than 80% of the total reinforcing filler.
 53. Theprocess according to claim 32, wherein the reinforcing inorganic filleris used in a mixture with carbon black.
 54. The process according toclaim 53, wherein the carbon black is present in an amount of between 2and 15 phr.
 55. The process according to claim 54, wherein the carbonblack is present in an amount of between 4 and 12 phr.
 56. The processaccording to claim 38, wherein the coupling agent is selected from thegroup consisting of bis-(C₁-C₄)alkoxylsilyl(C₁-C₁₀)alkyl polysulphides.57. The process according to claim 56, wherein the coupling agent is abis-(C₁-C₄)alkoxylsilylpropyl polysulphide.
 58. The process according toclaim 57, wherein the coupling agent is bis 3-triethoxysilylpropyldisulphide or tetrasulphide.
 59. A tire comprising a tread comprising arubber composition based on: (i) a diene elastomer; (ii) more than 50phr of an inorganic filler as reinforcing filler; (iii) between 2 and 15phr of an inorganic filler-diene elastomer coupling agent; (iv) between1 and 10 phr of a methylene acceptor, and (v) between 0.5 and 5 phr of amethylene donor.
 60. The tire according to claim 59, wherein the dieneelastomer is selected from the group which consists of polybutadienes,synthetic polyisoprenes, natural rubber, butadiene copolymers, isoprenecopolymers and mixtures of these elastomers.
 61. The tire according toclaim 59, wherein the reinforcing inorganic filler is a siliceous oraluminous filler.
 62. The tire according to claim 61, wherein thereinforcing inorganic filler is silica.
 63. The tire according to claim59, wherein the quantity of reinforcing inorganic filler is between 60and 100 phr.
 64. The tire according to claim 59, wherein the inorganicfiller-diene elastomer coupling agent is an organosilane or apolyorganosiloxane.
 65. The tire according to claim 64, wherein thecoupling agent is a polysulphurized alkoxysilane.
 66. The tire accordingto claim 59, wherein the quantity of methylene acceptor is between 2 and8 phr.
 67. The tire according to claim 59, wherein the quantity ofmethylene donor is between 0.5 and 3.5 phr.
 68. The tire according toclaim 59, wherein the methylene acceptor is selected from the groupconsisting of phenolic resins.
 69. The tire according to claim 68,wherein the methylene acceptor is a novolac phenolic resin.
 70. The tireaccording to claim 59, wherein the methylene donor is selected from thegroup consisting of hexamethylenetetramine (HMT),hexamethoxymethylmelamine (H3M), hexaethoxymethylmelamine, polymers ofpara-formaldehyde, N-methylol derivatives of melamine, and mixtures ofthese compounds.
 71. The tire according to claim 70, wherein themethylene donor is selected from the group consisting of HMT, H3M, andmixtures of these compounds.
 72. The tire according to claim 59, whereinthe quantity of methylene acceptor represents between 2% and 15% byweight relative to the weight of reinforcing inorganic filler.
 73. Thetire according to claim 59, wherein the quantity of methylene donorrepresents between 10% and 80% by weight relative to the weight ofmethylene acceptor.
 74. The tire according to claim 59, wherein thediene elastomer is a butadiene-styrene copolymer (SBR).
 75. The tireaccording to claim 74, wherein the SBR elastomer has a styrene contentof between 20% and 30% by weight, a content of vinyl bonds of thebutadiene fraction of between 15% and 65%, a content of trans-1,4 bondsof between 20% and 75% and a glass transition temperature of between−20° C. and −55° C.
 76. The tire according to claim 75, wherein the SBRis a SBR prepared in solution.
 77. The tire according to claim 59,wherein the reinforcing inorganic filler represents more than 80% of thetotal reinforcing filler.
 78. The tire according to claim 59, whereinthe reinforcing inorganic filler is used in a mixture with carbon black.79. The tire according to claim 78, wherein the carbon black is presentin an amount of between 2 and 15 phr.
 80. The tire according to claim79, wherein the carbon black is present in an amount of between 4 and 12phr.
 81. The tire according to claim 65, wherein the coupling agent isselected from the group consisting ofbis-(C₁-C₄)alkoxylsilyl(C₁-C₁₀)alkyl polysulphides.
 82. The tireaccording to claim 81, wherein the coupling agent is abis-(C₁-C₄)alkoxylsilylpropyl polysulphide.
 83. The tire according toclaim 82, wherein the coupling agent is bis 3-triethoxysilylpropyldisulphide or tetrasulphide.
 84. A tire comprising the tread accordingto claim
 29. 85. A tire comprising the tread according to claim
 30. 86.The tire according to claim 59, wherein said tire is a tire for avehicle selected from the group consisting of passenger vehicles, 4×4vehicles, motorcycles, vans and heavy vehicles.
 87. The tire accordingto claim 86, wherein said tire is a tire for a passenger-car.
 88. Thetire according to claim 86 or 87, wherein said tire is a “winter” tireintended for snow-covered or icy roads.